An electromagnetic actuator for actuating a cylinder valve in a piston-type internal-combustion engine essentially comprises two spaced electromagnets, whose pole faces face one another, and between which an armature that acts on the cylinder valve to be actuated is guided to move back and forth, counter to the force of at least one restoring spring, between an open position and a closed position for the cylinder valve. One of the electromagnets serves as a closing magnet, by means of which the cylinder valve is held in the closed position, counter to the force of the opening spring, while the other electromagnet serves as an opening magnet, by means of which the cylinder valve is held in the open position by way of the armature, counter to the force of the associated closing spring.
The arrangement is such that, in the resting position, the armature assumes a center position between the two pole faces. When the two electromagnets are alternately supplied with current, the armature comes into contact with the pole face of the respective supplied, and therefore capturing, electromagnet, counter to the force of a restoring spring. If the retaining current to the retaining electromagnet is turned off, the force of the restoring spring accelerates the armature in the direction of the other electromagnet, which is acted upon with a correspondingly high capturing current during the armature movement, so after overshooting the center position, the armature comes into contact due to the magnetic force, counter to the force of the restoring spring associated with the respective capturing electromagnet.
The electromagnetic actuator is controlled as a function of the operating data of the piston-type internal-combustion engine, essentially the load requirement and the rpm, which are available to the engine control unit. If the cylinder valve is, for example, in its closed position, i.e., the armature rests against the closing magnet, the control is a function of time - in other words, the engine control unit effects the control with consideration of the crankshaft position and the parameters from the load specification, which respectively determine the opening and closing times for the cylinder valve. The turn-off of the relatively low retaining current initiates the beginning of the armature movement, so the capturing current to the capturing electromagnet can be turned on after a predeterminable delay following the turn-off of the retaining current to the capturing electromagnet. The delay can be determined by way of previous empirical data, or theoretical data.
The time of the turn-off of the retaining current must be determined precisely, but is not identical to the time of the beginning of the armature movement, because the electromagnetic processes, such as the slow breakdown of the retaining magnetic field, and external influences, such as gas counterpressure counter to the cylinder valve to be opened, frictional resistances, etc., result in a so-called xe2x80x9csticking timexe2x80x9d for the armature. The actual armature movement therefore does not begin until after a specific delay following the turn-off of the retaining current.
If the capturing current is now initiated, as the armature continues to approach the pole face of the capturing electromagnet with a constant current supply, the magnetic force increases progressively, whereas the force of the restoring spring acting in the opposite direction only increases linearly. Consequently, the armature accelerates increasingly in the end phase, shortly before impacting the pole face of the capturing electromagnet, so the armature impacts the pole face hard, which is undesirable for several reasons: It produces physical or airborne sound and consequently promotes the development of noise. To avoid this, an appropriate control of the capturing current is aimed at reducing the current shortly before the armature impacts the pole face of the respective capturing electromagnet; a sensor element is used to detect the armature approach. This can be effected in that, when a predetermined armature position in the vicinity of the pole face is reached, a corresponding control signal is emitted, or the armature movement in this region is detected. The engine control unit, or a separate current control for the actuator, can use these approach values for the actuator for reducing the capturing current such that the armature impacts the pole face gently, i.e., at a speed only slightly greater than xe2x80x9czero,xe2x80x9d so the impacting electromagnet is only acted upon by the low retaining current.
These known control means are, however, inflexible, and do not take into account the numerous external interfering forces that act on the system comprising the armature and the cylinder valve, on the one hand, and on the other hand, they do not sufficiently minimize the development of noise.
It is the object of the invention to create a method that permits a much more precise control of an electromagnetic actuator.
In accordance with the invention, this object is accomplished by a method for controlling an electromagnetic actuator for actuating a cylinder valve in a piston-type internal-combustion engine, having two spaced electromagnets, between which an armature acting on the cylinder valve is guided to move back and forth between the pole faces of the two electromagnets, counter to the force of at least one restoring spring, with the electromagnets being alternately acted upon with a capturing current by way of a control, and with a sensor element detecting the movement of the armature on its path from the one pole face to the other pole face, specifically such that, in a first phase beginning with the initiation of the detachment of the armature from the pole face of the retaining electromagnet, the sensor element detects the actual values of the armature movement; in a second phase, as a function of the detected actual values of the armature movement, the control actuates the capturing electromagnet with regard to the current supply such that the armature is moved, at a predetermined speed and an acceleration approaching xe2x80x9czero,xe2x80x9d in a predeterminable spacing range from the pole face of the capturing magnet; and in a third phase, the current supply to the capturing electromagnet is controlled such that the armature impacts the pole face at a predeterminable minimum speed. The xe2x80x9cinitiation of the detachment of the armaturexe2x80x9d is defined by the time of the turn-off, preferably the purposeful reduction, of the retaining current. The term xe2x80x9cactual values of the armature movementxe2x80x9d encompasses not only the time of the turn-off of the retaining current in the first phase, but also the respective position, speed and acceleration of the armature in at least the first and second phases. Depending on the nature of the sensor element, in addition to the armature position, the armature speed can be detected directly, or, like the acceleration, derived from the path derivation over time, which results from the detection of position.
The division of the armature movement into three phases takes into account the physical qualities of the actuator, namely its individual mechanical qualities and the qualities that change over the course of operation of the piston-type internal-combustion engine. In the first phase, the armature movement is only xe2x80x9cobserved,xe2x80x9d during which the energetic initial position of the armature movement is detected, the position being essentially predetermined by the actual time of the detachment from the pole face and by the force of the restoring spring that accelerates the armature, as well as by the counteracting frictional forces and gas-pressure forces. When the armature is detached, unavoidable energy losses in the mechanical system occur in the vicinity of the electromagnet due to the residual field acting in the opposite direction. These negative electromagnetic force influences can be minimized through the use of a low-eddy-current armature and/or the supply of a current of a different polarity, which generates a magnetic field that has a repelling effect on the armature.
As soon as the armature has perceptibly detached from the pole face of the previous retaining electromagnet, it is no longer possible to influence the armature, either through a corresponding supply of current to the previous retaining electromagnet or through a premature supply of current to the capturing electromagnet with a current intensity that is justifiable from the standpoint of the power outlay. The armature travels at its highest speed when passing through the center position. In this region, other influences, such as internal cylinder pressure, frictional influences or other actuator parameters, can impact the armature movement, but are scarcely influenced by the magnetic force. Therefore, at low rpms of the piston-type internal-combustion engine, and correspondingly low movement speeds of the cylinder valves and thus of the armature, it is particularly advantageous not only to control the current supply to the capturing electromagnet. The purposeful guidance of the current supply to the releasing electromagnet, instead of a simple turn-off of the current, permits the influencing of the course of armature movement in this phase as well, and the forcing of a predetermined course of movement at the beginning of the movement.
If, as provided in the method of the invention, the sensor element detects the actual values of the armature movement in the first and second phases, it is possible for the respective interferences that act on the armature in the first phase, and are essentially caused by detachment processes and external influences, such as the internal cylinder pressure to be overcome, and the interferences that are essentially caused external influences in the second phase, to be supplied as control signals to the control or the individual actuator control; it is also possible to actuate the capturing magnet, with regard to the current supply, in the second phase such that the armature moves at a predetermined speed and an acceleration that approaches xe2x80x9czeroxe2x80x9d in a predeterminable spacing range, a so-called xe2x80x9ctarget window.xe2x80x9d This permits an individual adaptation of the current supply of the respective capturing electromagnet, but also of the electromagnet that releases the armature, with the consideration of the external interfering influences acting on the armature during the first two phases of the movement. This is sufficient if the specifications with regard to speed and acceleration are met in the spacing range at the releasing and the capturing electro magnet, because it is possible to purposefully guide the beginning and end of the armature movement in the immediate effective region of the magnetic fields of these electromagnets by correspondingly controlling the current supply.
The third phase, which begins when the target window is reached, is characterized by a low armature speed and a high force effect of the capturing magnet. In this phase, therefore, the current supply to the capturing electromagnet permits a controlled guidance of the armature counter to the force of the restoring spring until it comes to rest against the pole face, which assures a minimum impact speed.
The detection of the actual values of the armature movement in the first and second phases also permits the presetting of the spacing range with a corresponding actuation such that, instead of the armature impacting the pole face, it can be held to hover at a predeterminable distance from the pole face if, for example, it is undesirable for the armature to reach the end position due to time constraints, as is the case in a so-called free-fall actuation.
Because a controlled influence of the armature movement is possible in the third phase, if valve play exists, it is also possible to close the valve gently for setting the armature gently down on the pole face of the capturing magnet after it has detached from the valve.
In an advantageous embodiment of the invention, it is provided that the spacing range is predetermined as a function of the actual values of the armature movement that are detected at least in the second phase. It can be advantageous for the regulator associated with the actuator to be embodied as a model-based regulator, so the behavior of the system comprising the armature and the cylinder valve can be predicted.
It is especially advantageous for the current supply to the electromagnet to be controlled by way of a control of the voltage applied to the capturing magnet. A voltage control instead of a current control allows the necessary control efforts to have a far faster and more precise effect, because the current drops relatively slowly after the voltage turn-off, and, accordingly, the current increases relatively slowly when a voltage is applied. Because such electromagnetic actuators are usually acted upon with a direct current, it is also possible to brake an armature that is approaching the target window too rapidly through a brief generation of a counterfield by inverting the voltage at the end of the second phase, so the required values are attained in the target window. The voltage inversion is advantageously effected between an operating voltage, the dead setting (freewheeling, short-circuit) and a negative operating voltage (rear feeding).
An increased positive and negative voltage can compel a rapid change in current. The reversal can be effected very quickly. The voltage and current supply are advantageously drawn from the on-board network of the piston-type internal-combustion engine.
It is particularly advantageous when a sensor element having digital signal detection and signal processing detects the actual values of the armature movement. Such a sensor element can directly tap, for example, the position, that is, the path and/or speed at the armature or a guide rod connected to the armature, the rod being embodied as a digital path indicator, so very finely-divided signals that are tapped directly at the armature are available. The method can, however, also be realized with an analog or analog/digital sensor element.